1. Field of the Invention
The present invention relates to a cathode active material, a cathode electrode and a non-aqueous secondary battery. More particularly, the present invention relates to a cathode active material and a cathode electrode that give a non-aqueous secondary battery excellent in cycle characteristics and charge/discharge characteristics as well as to a non-aqueous secondary battery excellent in cycle characteristics and charge/discharge characteristics.
2. Description of the Related Art
As a secondary battery for a portable electronic apparatus, a non-aqueous secondary battery such as a lithium secondary battery is put into practical use and is widely prevalent. Further, in recent years, a lithium secondary battery is attracting people's attention not only as a small one for a portable electronic apparatus but also as a large-capacity device for being mounted on a vehicle or for electric power storage. For this reason, there is an increasing demand for safety, cost performance, lifetime and the like.
A lithium secondary battery has a cathode electrode, a anode electrode, an electrolyte and a separator as principal constituent elements thereof. Also, the cathode electrode is constituted of a cathode active material, an electroconductive material and a binder (binding agent).
Generally, a layered transition metal oxide represented by LiCoO2 is used as the cathode active material. However, the layered transition metal oxide is liable to provoke oxygen elimination in a fully charged state at a comparatively low temperature around 150° C., and this oxygen elimination can provoke thermal bursting reaction of the battery. Therefore, when a battery having such a cathode active material is used in the portable electronic apparatus, there is a fear that an accident such as heat generation or fire catching of the battery may occur.
For this reason, in view of safety, lithium manganate (LiMn2O4) having a spinel structure, lithium iron phosphate (LiFePO4) having an olivine structure and the like that are stable in structure and do not release oxygen at an abnormal time are expected.
Also, in view of cost performance, cobalt (Co) has a problem of having a low degree of presence in the earth crust and being expensive. For this reason, lithium nickelate (LiNiO2) or a solid solution thereof.
(Li(Co1−xNix)O2), lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4) and the like that do not contain cobalt or have a small content of cobalt are expected.
Also, in view of lifetime, layered transition metal oxides have a problem of causing destruction of the structure of the cathode active material by intercalation and deintercalation of Li to and from the cathode active material accompanying charging/discharging. For this reason, because of being stable in structure, lithium manganate (LiMn2O4) having a spinel structure, lithium iron phosphate (LiFePO4) having an olivine structure and the like are expected rather than the layered transition metal oxide.
Therefore, as a cathode active material of a battery considering safety, cost performance, lifetime and the like, the above-described lithium iron phosphate having an olivine structure, for example, is attracting people's attention. However, when lithium iron phosphate having an olivine structure is used as a cathode active material in a battery, there will be a problem of lowering of the charge/discharge characteristics such as insufficient electron conductivity and low average electric potential.
For this reason, for the purpose of improving the charge/discharge characteristics, a cathode active material represented by the general formula AaMb(XY4)cZd (wherein A is an alkali metal; M is a transition metal; XY4 is PO4 or the like; and Z is OH or the like) is proposed (for example, see Japanese Unexamined Patent Publication No. 2005-522009: Patent Document 1).
Also, a cathode active material represented by the general formula LiMP1−xAxO4 (wherein M is a transition metal; A is an element having an oxidation number ≦+4 (“≦” means “<” and “=”); and 0<x<1) in which a P-site is substituted with the element A is proposed (for example, see Japanese Unexamined Patent Publication No. 2008-506243: Patent Document 2).
Also, as a cathode active material excellent in charge/discharge characteristics at a large electric current, a material represented by the general formula Li1−xAxFe1−Y−ZMyMezP1−mXmO4−nZn (wherein A is Na or K; M is a metal element other than Fe, Li and Al; X is Si, N or As; Z is F, Cl, Br, I, S or N) (for example, see Japanese Unexamined Patent Publication No. 2002-198050: Patent Document 3) and, as an electrode active material being economical at the time of production and having a good charging capacity and good rechargeability over multiple cycles, a material represented by Aa+xMbP1−xSixO4 (wherein A is Li, Na or K and M is a metal) (for example, see Japanese Unexamined Patent Publication No. 2005-519451: Patent Document 4) are proposed.
Further, lithium transition metal phosphorus such as LiFePO4 in which a difference in molar volume between at least two coexisting phases containing a lithium-rich transition metal phosphate phase and a lithium-poor transition metal phosphate phase is about 5.69 is proposed (for example, see Table 2 of International Publication No. 2008/039170: Patent Document 5).
Also, it is proposed to restrain the volume change ratio with a material represented by the general formula LiFe1−xMxP1−yXyO4 (wherein M is Zr, Sn, Y or Al) (for example, see International Publication No. 2010/134579: Patent Document 6). In an example of Patent Document 6, a cathode active material in which M contains Zr is disclosed.